1. Field of the Invention
The present invention relates generally to Electronic Surveillance Measurement (ESM) receivers and more particularly relates to the use of phase receivers in determining frequency and angle-of-arrival (AOA) in ESM receivers.
2. Description of the Prior Art
Electronic Surveillance Measurement (ESM) receivers commonly require that both frequency and angle-of-arrival (AOA) calculations be performed on input signals from targets of interest. Usually, frequency is measured first, with the measurement spanning a wide instantaneous bandwidth to maximize probability of signal intercept. After a signal of interest is detected and its frequency is determined, the bandwidth of measurement is narrowed substantially for the determination of AOA. Centering a narrow measurement band around the signal frequency improves signal separation in a multi-signal environment, and reduces the influence of receiver noise on the accuracy of the AOA measurement. Prior art uses separate systems to measure each of these two parameters. The frequency of the input signals is often measured using an Instantaneous Frequency Measurement (IFM) device as illustrated in FIG. 1A. The AOA is typically measured using an interferometer as illustrated in FIG. 1B. Both the IFM and the interferometer use the difference in phase between the received input signals to calculate the frequency and the AOA of the input signals.
The IFM illustrated in FIG. 1A includes a receptor element 10, a delay line 12 and an N-channel phase receiver 14. The input signals 16 are received from the target by the receptor element 10, such as an antenna. The received input signals 16 are then applied to the delay line 12, which provides two or more output signals delayed in time and thus relative phases, to the N-channel phase receiver 14. The frequency of the received input signals 16 are determined by the difference in phase between inputs to the N-channel phase receiver 14 by means well known in the art. Phase receivers are alternatively referred to as phase discriminators, phase correlators or quadrature mixers. Further detail regarding phase receivers is presented in the product specification catalog entitled Anaren RF & Microwave Components, February 1997, distributed by Anaren Microwave, Inc., 6635 Kirkville Road, East Syracuse, N.Y. 13057, which is hereby incorporated by reference in its entirety.
The interferometer illustrated in FIG. 1B includes the receptor elements 10 (such as antennas), and the N-channel phase receiver 14. The receptor elements 10 are offset by a predetermined distance d. The interferometer uses the difference in phase between the input signals received by the offset receptor elements 10 to determine the AOA. The sine of the AOA .theta. of the input signals 16 is proportional to the phase difference between the input signals received by the offset receptor elements 10 in accordance with equation (1) as follows: EQU Phase difference=2.pi. sin.theta. d/.lambda. (1)
Thus, both the interferometer and the IFM utilize N-channel phase receivers as a means for calculating the AOA and the frequency of the input signal, respectively.